**4.1 Laser photoacoustic spectroscopy: basic principles**

Laser photoacoustic spectroscopy is based on the photoacoustic effect that occurs at the interaction between light and matter with the generation of a sound wave. In 1880, Alexander Graham Bell discovered these phenomena [83] while trying to find wireless communication. Thus, he discovered that certain optically absorbing solids emit a sound when illuminated by a modulated light. In 1881, Bell [84] Tyndall [85], Röntgen [86] and Preece [87] have demonstrated that the photoacoustic (PA) effect occurs not only in solid but also in liquid and gas. They found also, that the sound was stronger when the sample was placed in a cavity called photophone or spectrophone. With the appearance of sensitive microphones, increased interest in this technique. Afterwards, techniques based on this phenomenon have been known a continuous development, and today can be applied in almost all disciplines of Science and Technology.

An instrument based on the PA effect and which uses a laser as a radiation source, has important advantages for the analysis of gas traces such as high sensitivity ppb or even ppt (parts per trillion) concentrations and selectivity, high dynamic range, high accuracy and precision, good time resolution, versatility, reliability, robustness and is easy to use.

Over the years, photoacoustic spectroscopy (PAS) has proven its ability to detect traces of gas and has been used successfully as a gas sensor in biological and medical applications [88–93].

In gases, the PA effect is produced as a result of the following sequences (see **Figure 1**) [76]: absorption of incident laser radiation modulated in frequency or amplitude by the target gas molecules; local heating due to non-radiative relaxation; the extension and contraction of the gas sample that determines the pressure variation, which is an acoustic wave; detection of acoustic waves using microphones.
